Maybe an ancient sage in Naruto's own world who has long - been in seclusion. This sage has knowledge of different power systems beyond just chakra. He notices Naruto's pure heart and strong will and decides to pass on his knowledge of haki to Naruto, believing that Naruto can use it for the greater good of the ninja world.
Well, in such a fanfic, Naruto could start learning haki perhaps from a mysterious traveler who has knowledge of different worlds. He might first struggle with understanding the concept of haki, like differentiating between Observation Haki and Armament Haki. Then, as he trains, he could gradually incorporate haki into his existing jutsu. For example, using Armament Haki to strengthen his Rasengan, making it even more powerful and capable of breaking through tough defenses that normal Rasengan couldn't. This would make him an even more formidable opponent in battles against tough enemies like Madara or Kaguya.
In some fanfictions, Naruto might discover haki during a particularly intense battle. Maybe when facing an enemy with an unusual power that his regular jutsu can't handle, he unknowingly taps into his latent haki ability.
In 'naruto haki fanfic', Haki can change the Naruto universe by adding a new layer of power. For example, it can make battles more complex as characters now have to consider both chakra and Haki. It might also change the way characters interact with each other, as those with Haki could sense things differently.
I think Conqueror's Haki would suit Naruto. He has that natural leadership and strong will, just like characters in One Piece who possess Conqueror's Haki. It would be a great addition to his existing confident and determined personality.
In a fanfic where Naruto uses haki, it could change his relationships with other characters too. For example, if he shows this new power, some characters might be in awe of him, while others might be jealous. His training to master haki could also lead to new interactions with mentors or allies who are familiar with this power. Also, his use of haki could potentially attract the attention of new enemies who are interested in this unique ability.
In this kind of fanfiction, usually the plot might involve Luffy starting to discover his latent conqueror's haki power. Maybe he first experiences some intense battles where he unknowingly shows signs of it. Then, he might meet an old and wise character who recognizes this power in him and starts to guide him on how to control and use it effectively.
Perhaps Naruto discovered a hidden power within himself that was similar to Busoshoku Haki. In the fanfic, the author might have created a new backstory for Naruto where his Uzumaki heritage had some latent abilities that could be manifested as Haki - like powers. So, he just had to figure out how to use it on his swords.
One great 'naruto haki fanfic' is 'Naruto: The Haki Awakening'. It combines the powers of Haki from One Piece into the Naruto universe in a really interesting way. The main character's journey to master Haki while also dealing with the typical Naruto - style challenges is exciting.
The Naruto learns Rasengan fanfic can take the story in various directions. One direction could be a more in - depth exploration of the Rasengan's origin within the context of the fanfic. Another could be a time - travel plot where Naruto goes back in time and teaches his younger self how to learn Rasengan faster. It might also focus on the emotional turmoil Naruto goes through when he struggles to learn the jutsu, especially if he feels like he's letting down his village or his teacher Jiraiya.